1. Field of the Invention
This invention generally relates to mobile communication systems, and more particularly to a method and apparatus for recognizing mobile signals in a receiving base station.
2. Description of the Related Art
The first mobile communication system was provided in Saint Louis of the USA in 1946 using 150 MHz of frequency bandwidth. Systems of this type have developed into various complicated structures to enable users to talk over the phone with anybody, anytime, anywhere. Seeing the value in there systems, the number of subscribers of mobile communication systems has tremendously increased, especially in recent years.
There are increasing demands for fast, high-quality, mobile communication services. However, general frequency division multiple access (FDMA) or time division multiple access (TDMA) are not satisfactory due to limited frequencies. In order to solve the foregoing problems, CDMA has been introduced. CDMA provides many technical benefits such as large call capacity and high-quality call services. Even with these improvements, there are strong demands for better services. In terms of quality of service, users demand fast and stable services for mobile terminals, especially ones moving at high speed
User equipment (UE) registered in mobile communication systems allow users to communicate with anyone, anytime, anywhere. However, UEs have technical defects. For example, when a user moves at a high rate of speed in a car, unit time data rate tends to decrease and bit error rate (BER) tends to increase, thereby generating noise and/or call interruptions calls. In an attempt to overcome these drawbacks, a number of related technologies have been developed and are still being researched.
A mobile communication base station must always receive constant-level signals to provide stable services regardless of whether UEs are moving at a high or low speed rate of speed. The moving speed of each UE is decided according to the Doppler effect. UE signals received by a base station modem include unnecessary signals or noise.
Communication signals carrying information are digitally transmitted in a CDMA mobile communication system, and received signals have noise. A receiving end uses a method for accumulating a plurality of constant window unit signals in a synchronous status in order to separate pure communication signals from noise.
In a method which accumulates window signals having the same phase, when adjacent signals are presumed to have the same phase, an accumulated level increases according to the square of the sum of the accumulated signal sizes. When various types of noise are present, they have different phases and thus can be partially offset. The principle that the size of accumulated noise becomes the sum of the square of noise sizes is used in this case. For example, when signals are 5 and noises are 5 (namely at the ratio of 1 to 1), the accumulated signals are (5+5)2=100 and the accumulated noises are 52+52=50, namely the ratio of 2 to 1 according to the above accumulation. That is, this method improves a signal-to-noise ratio (SNR).
Signals often accumulated in slot units (often called coherent multi-slot accumulation) are also used to improve SNR. Another method known as non-coherent multi-slot accumulation accumulates energy of signals in slot units to increase energy of signals. In these methods, when the UE moves at high speed, a weight is applied to the non-coherent multi-slot accumulation. Conversely, when the UE moves at low speed, a weight is applied to the coherent multi-slot accumulation.
A method for recognizing signals in a multi-path searcher of a general base station modem will now be explained with reference to the accompanying drawings.
FIG. 1 illustrates a total searching process time for an accumulation slot number Lslot according to the non-coherent multi-slot accumulation, specifically a real non-coherent multi-slot accumulation process for receiving and accumulating signals of the UE moving at high speed and with increasing energy. In this figure, when the accumulation slot number Lslot is 4, non-coherent multi-slot accumulation is performed with four kinds of slots: T slot, R slot, C slot, and S slot.
The T (Throw process) slot is a task processor area for recording data values for mobile communication of the UE. The T slot is a slot for reading parameter information such as subscriber numbers, pseudo noise (PN) code values, and synchronous position values, and has a constant time period. The R (Run process) slot is a slot for calculating energy values of signals. The C (Catch process) slot is a slot for temporarily storing the resultant values. And, the S (Sort process) slot is a slot for sequentially sorting the energy values calculated in the R slot.
As shown, the T slot is processed below the C slot. The C slot is not a processing process but a temporary storing process, and thus can perform another process at the same time. Accordingly, the T slot is processed during the C slot. In addition, when the accumulation number is 4, the T, R and C slots are repeated four times and then the S slot is processed. A total of 10 slots of signal-searching process time is taken, as represented by following formula:Non-coherent searching process time=[2(1+Lslot)] slot
FIG. 2 illustrates a total searching process time for an accumulation slot number Mslot according to the coherent multi-slot accumulation for receiving and accumulating signals of a UE moving at low speed, for example, when the accumulation slot number Mslot is 4. As shown, the searching process performed using the coherent multi-slot accumulation includes four slots such as T slot, R slot, C slot and S slot. However, different from non-coherent multi-slot accumulation, the C and T slots are not processed at the same time, the R slot is repeated four times after the first T slot, and the C and S slots are sequentially processed.
Also during this searching process, a total of 7 slots of signal searching process time is taken, as represented by following formula:Coherent searching process time=[3+Mslot]slot
When the same number of slots are accumulated, the coherent multi-slot accumulation shows better properties than non-coherent multi-slot accumulation. As a result, it is advantageous in a mean acquisition time which is an average searching process time for recognizing received signals in the multi-path searcher of the base station modem.
FIG. 3 shows an apparatus for recognizing signals in a multi-path searcher of a base station modem using coherent and non-coherent multi-slot accumulation functions, and also using a Doppler estimator for deciding an accumulation slot number. The apparatus for recognizing signals in the multi-path searcher of the base station modem includes a scrambling code generator 10, a despreader 20, a first coherent accumulator 30, a second coherent accumulator 40, a first squaring circuit 50, a second squaring circuit 60, an adder 70, a non-coherent accumulator 80, a memory 90 and a Doppler estimator 100.
The despreader 20 applies a scrambling code signal to an I-channel signal r1 and a Q-channel signal rQ, received from an antenna and divided into an in-phase (I) channel and a quadrature-phase (Q) channel, to despread the signals. The scrambling code generator 10 generates the scrambling code signal and transmits the signal to the despreader 20. The first and second coherent accumulators 30 and 40 multiply the output signals from the despreader 20 by a pilot signal, and accumulate the resulting signals. The first and second squaring circuits 50 and 60 square each of the accumulated signals and extract size elements of the signals. The adder 70 adds up the output signals from the first and second squaring circuits and outputs the resultant value. The non-coherent accumulator 80 accumulates signal sizes of the signals from the adder. The memory 90 sequentially stores the output signals from the non-coherent accumulator. The Doppler estimator 100 measures the moving speed of the UE and transmits a control signal proportional to the moving speed to the first and second coherent accumulators 30 and 40 and the non-coherent accumulator 80.
FIG. 4 is a flowchart showing sequential steps of a process for searching signals performed by the aforementioned apparatus for recognizing signals in the multi-path searcher of the base station modem. As shown, a signal received by the base station is divided into a complex type I-channel signal r1 and Q-channel signal rQ through a transmission channel (S10). The despreader receives the I-channel signal r1 and Q-channel signal rQ, and applies a scrambling code from the scrambling code generator for spreading to the I-channel signal r1 and Q-channel signal rQ, thereby despreading the signals (S11).
The Doppler estimator receives the signals from the base station antenna analyzes and processes the received signals according to the Doppler effect, to decide the moving speed of the UE (S17), and outputs a control signal based on the moving speed under the preset standards. When the analyzed moving speed is high, the Doppler estimator outputs a control signal for adding a weight to Lslot to increase an accumulation slot number in the non-coherent accumulator (S18), and when the analyzed moving speed is low, the Doppler estimator outputs a control signal for adding a weight to Mslot to increase an accumulation slot number in the coherent accumulator (S19).
The first and second coherent accumulators receive the despread signals from the despreader and multiply the signals by pilot patterns. The resulting symbol unit signals become Npilot pilot symbols. The first and second coherent accumulators accumulate the whole pilot symbols in the pilot symbol period, and also accumulate signals in the Mslot-1 slot pilot symbol period according to the given Mslot number (S12). The first and second squaring circuits square the signals accumulated for the predetermined time, and extract size elements E1 and EQ of the signals (S13). The adder adds up the square elements of the I and Q signals from the first and second squaring circuits, to obtain one energy Etotal (S14). Here, the energy value is a signal size in that phase. Thereafter, the non-coherent accumulator accumulates the energy signal Etotal from the adder by the given Lslot (S15). The memory sequentially stores the output signals from the non-coherent accumulator as searching energy values (S16).
In this process, the signals before the adder are pairs of I and Q signals, and each of the blocks is embodied in pairs (S10˜S13). However, the Doppler estimator merely considers the moving speed of the UE. Even if the moving speed of the UE is high, if the SNR is low, performance is not improved in the non-coherent multi-slot accumulation.
Moreover, a mean acquisition time which is an average time taken to recognize received signals in the base station modem is longer in the non-coherent multi-slot accumulation than the coherent multi-slot accumulation. Accordingly, it is meaningless to give the weight to the non-coherent accumulator.
In addition, an initial synchronization time may be delayed longer and wireless resources of the base station may be spent wastefully. That is, the Doppler estimator which merely considers the moving speed of the UE in giving the weight to the coherent accumulator and the non-coherent accumulator is inefficient.